The invention relates to nuclear medicine, and more particularly relates to nuclear medicine studies in which more than one isotope is administered to the patient. In its most immediate sense, the invention relates to dual-isotope myocardial perfusion studies.
It has long been known to administer e.g. two radioisotopes to a patient and to form two images of the patient, one relating to one isotope and the other relating to the other isotope. U.S. Pat. No. 3,904,530 teaches apparatus which is designed to be used in such a study. Presently, the only significant clinical application of a dual-isotope study is a parathyroid study. Such a study is designed to detect the existence of an adenoma (which takes up Tl) on the thyroid (which takes up both Tc and Tl). In this study, subtraction of the Tc image from the Tl image will reveal the adenoma.
Recently, Daniel S. Berman, M.D. at the Cedars-Sinai Medical Center (Los Angeles, Calif.) has proposed to improve existing myocardial perfusion studies by using a dual-isotope technique and utilizing a recently developed Tc-99m sestamibi pharmaceutical marketed by Du Pont Merck Pharmaceutical Co. (Wilmington, Del.) under the CARDIOLITE trademark. The present invention has particular utility in such studies and to understand this utility it is necessary to understand the purposes and protocols used in myocardial perfusion studies. This will now be explained.
There are two major categories of cardiac problems. One category includes patients who have cardiac infarctions (dead myocardial tissue). The other category includes patients who have arteriosclerosis (disease of the coronary arteries). Clinically, it is necessary to distinguish between these two categories because they are managed differently. (When a cardiac patient is shown to have an infarcted myocardium, no surgical intervention is used and the patient is medicated and instructed to avoid heavy physical activity. When a cardiac patient is shown to have arteriosclerosis, surgical intervention can be used to correct the problem.)
A myocardial perfusion nuclear medicine study is presently used to distinguish between these two categories of problems. In such a study, Tl (this is most commonly used, but Tc can also be used) is administered to a patient while the patient has reached the point of maximum effort in a stress test. (The stress test can be administered physically, in which case the patient is placed on a treadmill. Alternatively, for bedridden patients, the stress test can be administered pharmacologically by administering a dipyridamole drug sold under the I.V. PERSANTINE trademark by Du Pont Merck Pharmaceutical Co. This also increases blood flow to the heart muscle.) The Tl collects in the myocardium (heart muscle) and its initial distribution within the myocardium shows the regions where the myocardium is perfused with blood flow. Then, the patient's heart is imaged in a gamma camera to record this heart-under-stress blood perfusion. Afterward, the patient is permitted to rest so that the stress conditions entirely dissipate. As this happens, the Tl redistributes itself within the heart, and after a period of time (aided in some, but not all, instances by another injection of Tl) the distribution of Tl within the myocardium represents blood perfusion in the heart at rest. Then, the patient's heart is imaged a second time in a gamma camera to record this heart-at-rest blood perfusion.
If a region of the myocardium shows Tl uptake when the heart is at rest, this indicates that this region contains living tissue. If the same region of the myocardium then lacks Tl uptake when the heart is stressed, this indicates the presence of arteriosclerosis which prevents blood from reaching region under stress conditions. On the other hand, if a region of the myocardium shows no Tl uptake either at rest or under stress, this indicates that the region contains an infarction which blood does not perfuse.
This conventional myocardial perfusion protocol takes a long time. Typically, the study lasts an entire day. This is because each session in the camera lasts about an hour and the interval between sessions is four hours. Camera throughput is low and study cost is consequently high.
Dr. Berman's research has developed a study protocol which would drastically shorten the duration of such myocardial perfusion studies. According to this new study protocol, the resting patient is first injected with a Tl agent. In a few minutes, the Tl will be taken up by the heart in accordance with heart-at-rest blood perfusion. Thereafter, the patient is subjected to a stress test and injected, at peak stress, with the Tc CARDIOLITE agent. This agent too is taken up within a few minutes. Then, the patient is placed in a gamma camera and a dual-isotope study of the patient is conducted. Such a dual isotope study produces two images--a Tl image and a Tc image--of the patient's heart and the relationship between the images shows where heart muscle is affected by arteriosclerosis and where it is infarcted.
This new study protocol produces dramatic improvement in camera throughput, because the prolonged wait between injections is eliminated and only one camera session is needed instead of two. This is a major development because myocardial perfusion studies are the second most common application for nuclear medicine.
Conventional gamma cameras with dual-isotope capability are not able to make optimum use of this new study protocol. This is because there is a degree of crosstalk between the two acquisition channels, i.e. events from the Tc channel will be counted in the Tl channel and vice versa. In the case of parathyroid studies, this is not particularly disadvantageous, but in cardiac applications this would significantly degrade the diagnostic utility of the images produced. It would therefore be advantageous to provide gamma cameras with improved crosstalk characteristics so as to better benefit from this new study protocol.
One object of this invention is to provide method and apparatus which could be used in multi-isotope studies, particularly dual-isotope studies, wherein crosstalk between the resulting images would be reduced.
Another object is to provide method and apparatus which could be used to improve the images produced by a dual-isotope myocardial perfusion study.
A further object is to provide method and apparatus by which existing gamma cameras with dual-isotope capability could be retrofitted so as to be usable in dual-isotope myocardial perfusion studies.
Still another object is to improve on method and apparatus of this general type.